This invention relates to the continuous surveillance of sensors used for monitoring changes in the various parameters of a system. More particularly, the invention provides a method and apparatus for on-line detection of degradation or undesirable changes in the time constants of sensors used in monitoring the flow rate, temperature, pressure, level, radiation, etc., of a system.
Heretofore, evaluation and calibration of sensors used in monitoring a process or system, typically took place during periodic maintenance or when the system was completely shut down. However, as industrial processes have become more continuous and complex, it is often undesirable and very expensive to shut down an industrial process as often as would be necessary to provide adequate periodic calibration and evaluation of the sensors monitoring the process. In addition, some industrial processes and systems operations, including nuclear power plant operations, represent such potential danger that any degradation or reduced performance of sensors used in monitoring the process or system could be critical. Therefore, it is of great importance that any such degradation be brought to the attention of the person responsible for the systems opration.
The use of fluctuating output signals to obtain information about the dynamic characteristics of a system or process in itself is not new. For example, refer to "Random Data: Analysis and Measurement Procedures" by J. S. Bendat and A. G. Piersol which was published by John Wiley and Sons, Inc., New York, 1971; and "Random Noise Techniques in Nuclear Reactor Systems" by R. E. Uhrig which was published by the Ronald Press company, New York, 1970. Typically, these available methods as discussed in the above references include evaluation of spectral densities or time series models. According to these methods, however, it is necessary to perform extensive calculations on the raw data to obtain the desired results. In addition, methods have been devised which continually monitor and use the statistical properties of a sensor output to detect degradation. A discussion of some of the methods may be found in "Two On-Line Methods for Routine Testing of Neutron and Temperature Estimates of Power Reactors" by M. Eldemann, a Kernforschungszentrum Karlsruhe Report, KFK, 2316, July, 1976 Karlsruhe, Germany; and "In-Situ Response Time Testing of Platinum Resistance Thermometers" by T. W. Kerlin et al. a EPRI Report, NP-459, January, 1977, Palo Alto, Calif. Unfortunately, as was mentioned above, the presently available methods and apparatus are typically very complex and expensive and require such extensive calculation on the raw data that a computer is usually necessary for implementation. However, the previous methods and techniques were more powerful than necessary for simply monitoring the sensor response characteristics. Therefore, as will become clear hereinafter, the zero crossing method used in the sensor response time degradation monitor of this invention uses a technique which is, therefore, less demanding than the presently available methods and techniques which require a detailed computer analysis.